This invention relates to an impulse noise reducer based on digital signal processing, applicable to a medium-wave amplitude-modulation (AM) audio broadcast receiver.
Impulse noise such as ignition noise is a problem for medium-wave AM audio broadcast receivers in general, and especially for receivers installed in vehicles. Such receivers are therefore often equipped with impulse noise reducers that detect and remove impulse noise. Conventional impulse noise reducers detect impulse noise by processing a comparatively wide-band intermediate-frequency signal, and remove the detected impulse noise from the output audio signal. The reason for this arrangement is that impulse noise can be more easily discriminated from high-frequency audio components in the intermediate-frequency signal than in the audio output signal itself.
A consequent problem is that the impulse noise reducer requires its own intermediate-frequency signal-processing system, including, for example, an intermediate-frequency amplifier, a detector, and automatic gain-control circuitry. The structure of the conventional impulse noise reducer is correspondingly complex, requiring many extra circuit components. This is especially true if the impulse noise reducer has an analog circuit configuration, which has generally been the case in the past.
A further problem is that when a strong adjacent-channel broadcast signal is present, the gain of the intermediate-frequency amplifier in the noise-detection system is automatically reduced, reducing the noise detection sensitivity, so that much impulse noise goes undetected.
Due to the increasing density of digital integrated circuits, digital circuit configurations are becoming advantageous, both because of their smaller size and for compatibility with digital audio broadcast receiving circuits. A digital impulse noise reducer remains complex, however, if it must detect impulse noise from the intermediate-frequency signal, and there also remains the problem of reduced sensitivity when a strong adjacent-channel broadcast signal is present.
An object of this invention is to reduce the complexity of an impulse noise reducer.
Another object is to improve the performance of an impulse noise reducer.
A further object is to make an impulse noise reducer immune to the effects of adjacent-channel broadcast signals.
The invented method of reducing impulse noise includes the following steps:
(a) detecting the amplitude of a high-frequency component of an audio signal;
(b) smoothing the resulting amplitude signal;
(c) controlling the amplitude of the amplitude signal according to the smoothed amplitude signal, thereby obtaining a gain-controlled amplitude signal;
(d) comparing the gain-controlled amplitude signal with a threshold value, thereby obtaining a noise detection signal;
(e) determining a noise interval from the noise detection signal; and
(f) correcting the audio signal during the noise interval.
The complexity of this method is reduced because it detects impulse noise directly from the audio signal, and does not require extra intermediate-frequency signal processing. For the same reason, the invented method is immune to the effects of adjacent-channel broadcast signals, these adjacent-channel signals being substantially absent from the audio signal.
The method may include the further steps of extracting a low-frequency component of the amplitude signal, and adjusting the threshold value according to this low-frequency component, thereby improving the noise reduction performance by detecting impulse noise more accurately.
Step (c) may include the further steps of delaying the amplitude signal by an amount greater than a rising time constant occurring in step (b), and multiplying the delayed amplitude signal by a gain factor that decreases as the smoothed amplitude increases. This also improves the accuracy of impulse noise detection.
The method may include limitation of the amplitude signal before it is smoothed in step (b). Impulse noise detection is thereby improved when a strong noise impulse is followed by a weaker noise impulse.
Step (e) may include extending the interval during which impulse noise is indicated to be present by the noise detection signal. Impulse noise reduction performance is thereby improved when the noise detection signal only intermittently indicates the presence of a substantially continuous interval of impulse noise.
In this case, step (e) may extend the noise interval by a variable amount, depending on the amplitude of the high-frequency component of the audio signal, thereby improving the noise reduction performance for strong noise impulses.
Step (e) may extend the noise interval at both ends, so that initial parts of the noise interval are not missed.
This may be done by, for example, extending the stopping time of the interval by a predetermined amount, then advancing the start of the interval by an amount depending on a total length of the interval, enabling the extension process to be carried out in a simple way.
The invention also provides an impulse noise reducer implementing the invented method, and a medium-wave AM audio broadcast receiver incorporating the invented impulse noise reducer.